1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus and method for recording/reproducing information as hologram.
2. Description of the Related Art
Optical information recording media typically include a compact disk (CD), a digital versatile disk (DVD), a high-definition digital versatile disc (HD DVD), Blu-ray Disk, and the like. The optical information recording medium responds to an increase in recording density so far mainly by making a wavelength of a laser beam shorter and by increasing the numerical aperture (NA) of an objective lens. However, both the methods seem to be approaching the limit by some technical reasons, and it is therefore required to increase the recording density by other means and systems.
Recently, among various approaches, a volume-recording type high-density optical recording using holography (hereinafter, “holographic memory”) and a recording/reproducing device of the holographic memory (hereinafter, “holographic-memory recording/reproducing apparatus”) are being developed for practical use.
A recoding system of the holographic-memory recording medium is implemented in such a manner that an information beam and a reference beam are irradiated to one location of a recording medium to record therein interference fringes produced by the information beam and the reference beam upon irradiation. Specifically, the information beam carries information thereon by spatially modulating a laser beam using a spatial light modulator (SLM) such as a liquid crystal element and a digital micromirror device, and the reference beam with the same wavelength as that of the information beam is usually generated from the same light source as that of the information beam.
When information is reproduced from the holographic-memory recording medium, by radiating only the reference beat, the information beam upon recording is reproduced, to obtain information modulated at the time of recording. There is a so-called surface recording system such as a digital versatile disk (DVD) in which record marks are recorded on a recording surface. On the other hand, there is a holographic optical disc that is a volume recording system capable of recording information in the thickness direction of an information recording layer, and therefore the holographic optical disc can obtain higher recording density as compared with that of DVD.
In the case of DVD, the record marks generally indicate ON/OFF bit data, while in the case of the holographic-memory recording medium, the information beam is collectively modulated because of its comparatively large volume of information, to be recorded therein as interference fringes. This pair of information is a modulation pattern of the information beam held in the recording medium, and this modulation pattern is a unit of recording or reproduction of a two-dimensional barcode that consists of black and white dots, which is called page data.
One of methods of increasing the recording density of the holographic-memory recording medium is a multiple recording system. The multiple recording system is a system of recording a plurality of page data in one location of the holographic-memory recording medium, the system including various systems such as an angular multiplexing recording system of recording information while an irradiation angle of a laser bean is shifted and a shift multiplexing recording system of recording information while slightly shifting an irradiation position of a laser beam. A general multiplexing system is disclosed in, for example, a technical literature ‘H. J. Coufal, D. Psaltis, G. T. Sincerbox, “Holographic Data Storage”, Springer, 2000’.
In the angular multiplexing recording system and the shift multiplexing recording system, multiple recording is implemented by changing a relative angle or a relative position between a laser beam and a holographic-memory recording medium. The angular multiplexing recording system in particular is a system totally different from conventional disks such as a compact disk (CD) and a DVD, and is needed for a so-called two-light flux interference system in which interference fringes produced in a position of interference between the information beam and the reference beam are recorded in a medium recording layer.
When the angular multiplexing recording system and the shift multiplexing recording system are to be combined, it is considered that there are two typical technologies; a technology for causing a recording medium to rotate to perform angular multiplexing recording, and a technology for causing a laser beam to rotate around a recording medium to perform angular multiplexing recording. A rotation axis of the recording medium is generally set as an axis perpendicular to an incident plane of the information beam and the reference beam (or, an axis in a plane direction of the recording medium). It is considered that this setting can achieve a larger number of multiples than that upon rotation around some other axis. Hereafter, the rotation around the axis perpendicular to the incident plane of the information beam and the reference beam is called θy rotation, and multiple recording due to the rotation is called θy multiple recording.
The technology for causing not a laser beam but a recording medium to rotate to perform angular multiplexing recording has such features that the device configuration can be simplified because a movable unit for multiple recording does not need to be provided in an optical system such as lenses. However, this technology is difficult to be applied to a disk-shaped recording medium such as CD or DVD that rotates at a comparatively high speed. On the other hand, the technology for causing a laser beam to rotate around a recording medium to perform angular multiplexing recording has such characteristics that it is easily applied to the disk-shaped recording medium but the device structure becomes large.
Further, for example, U.S. Pat. Nos. 5,483,365 and 6,700,686, and JP-A 2004-354565 (KOKAI) disclose a technology for performing angular multiplexing in which two axes are used in such a manner that rotation around other axis is performed in addition to the θy rotation. This technology allows further improvement of recording density by performing angular multiplexing recording, in addition to the θy rotation, while rotating the recording medium or the laser beam around an axis perpendicular to a medium plane of the recording medium and passing through a recording spot. Hereafter, the rotation around the axis perpendicular to the medium plane and passing through the recording spot is called θz rotation, and multiple recording due to the rotation around the axis is called θz multiple recording.
However, there is an upper limit in a multiple number of the θz multiple recording, and it is known that there occurs crosstalk such that if the number exceeds a certain number, some data recorded at an adjacent angle comes in a reproduced image upon reproduction. This is because, unlike the θy multiple recording, the θz multiple recording has such characteristics that a range (maximum span) where recording can be performed is restricted to 180 degrees (half-turn) no matter how many rotations are made, and the intensity of a reproduction beam coming in from an adjacent page does not sufficiently decrease unless it is rotated quite largely.
The upper limit of the multiple number decreases as an increase in the numerical aperture (NA) of an objective lens through which the information beam passes. In the case of NA 0.65, the upper limit becomes about 2 to 3 depending on a form of page data, so that only 2 to 3 spots can be recorded every 90 degrees or 60 degrees in a rotation angle of 180 degrees. Therefore, the θz multiple recording requires any mechanism and adjustment or the like to accurately rotate the recording medium or the laser beam without displacement, which becomes a disadvantage in that the device structure becomes complicated. Consequently, the disadvantage may overweigh an advantage in that high recording density is achieved by using the θz multiple recording.